As everyone here has already noted; the little section at the end of the manual is helpful and of course, lots of "seat time" in the car will help.
But the best way to know which line to choose is to understand the principles involved in CHOOSING the correct line.
In real life racing, there is no such thing as "the perfect line". The best line for you and your car will vary depending on tires, car set up, race conditions and environmental conditions. There are dry lines and wet lines. A qualifying line may help you run fast, but in a race you may take a more defensive line that keeps the guy behind you from becoming the guy IN FRONT of you. As tires go away, you will need to adjust your braking and acceleration points... it all varies.
In GT3, the car you are driving and they way you have it set up has a big impact on what the best line may be.
There are some general rules to pass on, however:
1) a turn before a long straight is very important. Getting even a single mph coming out can translate into several extra car lengths by the end of the straight.
2) many corners on race tracks are compromise corners. That is, you will have to give up some speed on one in order to do well on the other.
3) good braking is just as crucial as getting good exit speed. This is especially true if you are braking at the end of a high speed straight. Late breaking and trail braking can take many tenths off your lap time.
4) just because your car "feels good" doesn't nessesarily mean you are going faster.
5) "slow in, fast out". Don't try to fly into every turn with your ABS pumping. A conservative corner entry can get your car rotated around for a earlier throttle application.
6) be smooth. I noticed that GT3 has a pretty good tire model. If you slam the car around, you're more likely to get it out of sorts and cross it up. Be gentle with your inputs.. especially on those powerful RWD cars.
I highly recommend a book called "Going Faster" by Carl Lopez, Danny Sullivan and the staff at Skip Barber Racing School. Its filled with good solid information and is a must read for any car nut.
///M-Spec